The issue of disposability of products is of great concern to the nonwovens industry. Among the different types of disposable nonwovens are pre-moistened nonwovens which are readily dispersible in large amounts of water or liquids, and absorbent, disposable nonwovens that can be used as wipes and require a high level of strength after contact with liquids.
With regard to dispersible nonwovens, the industry wishes to offer pre-moistened toilet tissue on a roll and pre-moistened wipes that will be truly flushable and dispersible. In another words, the tissues or wipes must disintegrate in toilet water under gentle agitation without the addition of temperature or chemicals.
Wet-packaged skin cleansing and refreshing tissues are well-known commercially, and generally referred to as towelettes, wet wipes, fem wipes and the like. These may comprise an absorbent sheet made of paper, prepared or treated to impart wet strength thereto, having the dimensions of the usual washcloth and packaged wet in folded condition, individually in impervious envelopes or in multiples in closed containers. The liquid employed in the pre-moistening of the wet-packaged bonded nonwoven web is generally an aqueous composition, or lotion, which may further contain a surface active agent and a humectant and, in some instances, a scenting agent. Instead of individual packaging of such moist sheets, they are often marketed in reclosable containers having any desired convenient number of such folded sheets.
Water soluble or redispersible polymeric binders have been used in making nonwoven substrates and they are generally comprised of poly(vinyl alcohol) (PVOH), PVOH-stabilized vinyl acetate or vinyl acetate based polymers, e.g., vinyl acetate-ethylene (VAE) emulsion polymers. Some of the technical problems associated with pre-moistened wipes and tissues include: (1) providing sufficient water soluble or dispersible binder in the nonwoven wipe to provide sufficient dry and wet tensile strength for manufacture and use in its intended application and (2) protecting the water redispersible polymer in the wipe or tissue from attack by the aqueous composition during storage.
Several mechanisms have been proposed to address disintegration of the pre-moistened wipe prior to use. One method employs the addition of boric acid or derivatives to the aqueous lotion composition to maintain the integrity of the wet nonwoven substrate. However, boric acid and its derivatives have fallen into disfavor due to the perception as to their potential harmful effects near mucous membranes.
Another mechanism for preventing dissolution of the wipe has been the inclusion of high levels of alkali metal bicarbonates and salts to prevent as the aqueous lotion from slowly dissolving the PVOH allowing this protective colloid to redisperse the polymer binder. A problem of alkali metal bicarbonates is that they can decompose slowly to carbon dioxide and evaporate out of the solution.
The following patents and articles are illustrative of the prior art:
U.S. Pat. No. 4,245,744 discloses nonwoven fiber sheets impregnated with PVOH-containing vinyl acetate-based polymers in which the nonwoven sheets are maintained in contact with a dilute aqueous solution of a precipitating or gelling agent for PVOH, such as boric acid.
U.S. Pat. No. 5,252,332 discloses a packaged towelette comprising a sheet of nonwoven fibers impregnated with PVOH or a PVOH-containing binder in contact with an aqueous solution containing boric ions and bicarbonate ions.
U.S. Pat. No. 5,629,081 discloses a pre-moistened, dispersible and biodegradable wet wipe comprising a web of nonwoven fibers contacted with a PVOH-containing binder. The binder-contacted web further comprises an aqueous lotion solution comprising 0.1-0.9 wt % boric acid and 5-8 wt % alkali metal bicarbonate, based on weight of the lotion.
U.S. Pat. No. 5,384,189 discloses a water decomposable nonwoven fabric in which the fibers are bonded to one another with a water-soluble binder comprising an unsaturated carboxylic acid/unsaturated carboxylic acid ester copolymer in which 1-60 mole % of the repeating units derived from the carboxylic acid is in the form of a salt. The binder is soluble in tap water but is insoluble in an aqueous solution containing not less than 0.5 wt % of a neutral inorganic salt comprising a monovalent ion.
U.S. Pat. No. 5,972,805 discloses a water-soluble polymeric binder composition for use in making nonwoven webs comprising 25-90 wt % unsaturated carboxylic acid/unsaturated carboxylic acid esters/ester copolymer, 10-75 wt % divalent ion inhibitor and 0-10 wt % plasticizer. The water-soluble binder composition is soluble in an aqueous environment having a divalent ion concentration listing about 50 ppm and a monovalent concentration of less than about 0.5 wt %.
U.S. Pat. No. 5,256,417 which discloses a packaged towelette which is disposable comprises a sheet of nonwoven fibers impregnated with a binder, e.g., PVOH or an aqueous polymer emulsion containing PVOH as a protective colloid. The sheet is maintained in a wet condition within the package by contact with a non-aqueous lotion composition which is a liquid organic compound that is a non-solvent for PVOH.
WO 99/18269 and WO 00/39378 disclose disposable products which are flushable. A feature of the flushable product is that it have sufficient wet strength for its intended use but loses its structural integrity upon contact with water. Ion sensitive, “temperature sensitive” compositions are employed in forming the disposable product and comprise a blend of at least one water-sensitive polymer and at least one polylactide, polyolefin-grafted with a polar group, e.g., maleic acid. Dispersiblity depends upon the amount of monovalent or multivalent ions in the aqueous solution.
Additionally nonwoven products are utilized for the preparation of absorbent, disposable products such as wipers. These nonwoven products or fabrics are also comprised of loosely assembled webs or masses of fibers bound together with an adhesive polymeric binder. These fibers have been comprised of cellulosic or polymeric materials such as polyesters, polyamides, polyacrylates and the like. However, these nonwoven products are expected to retain a high level of strength after contact with liquids. They are typically used to clean countertops, windows and many other hard surfaces. Many of the adhesive polymers for these nonwoven webs have been based upon water-based polymers of vinyl acetate, ethylene and a self crosslinking monomer, e.g., N-methylol acrylamide (NMA) or equivalent formed by emulsion polymerization The incorporation of a self crosslinking monomer provides for enhanced wet strength and resistance to organic solvents.
The following patents are representative of the art:
U.S. Pat. No. 3,380,851 discloses a nonwoven binder comprised of a polymer of vinyl acetate, another polymerizable compound as an internal plasticizer, and a post-curable comonomer such as N-methylol acryiamide bonded to loosely assembled fibers. A wide variety of fibers based upon natural and synthetic fibers which include cellulose, wool, polyamides, polyesters, polyethylene and so forth are disclosed.
U.S. Pat. No. 4,449,978 discloses nonwoven webs of the type disclosed in U.S. Pat. No. 3,380,851 bonded with low formaldehyde generating adhesive polymers. Specific systems are based upon vinyl acetate, ethylene and a blend of acrylamide and N-methylol acrylamide.
U.S. Pat. No. 3,758,429 discloses adhesive binders based upon terpolymers of ethylene, vinyl chloride and N-methylol acrylamide for use in producing nonwoven webs. The terpolymers are provided to enhance wet strength.
U.S. Pat. No. 3,752,733 discloses nonwoven products based upon fibers from both natural and synthetic sources bonded with an ethylene/vinyl chloride/acrylamide terpolymer. The patentees disclose curing these polymers with sulfur compounds to provide increased tensile strength, softness, flexibility and so forth.
U.S. Pat. No. 3,137,589 discloses binders comprising a copolymer of an alpha, beta-unsaturated carboxylic acid amide substituted on the nitrogen by at least one methylol group and another unsaturated polymerizable compound for use in preparing nonwoven webs.
Historically, to achieve desirable or sufficient wet tensile strength it has been common practice to elevate the dry tensile strength of the polymer or use higher add-on levels of polymer. However, the level of wet tensile typically plateaus at a performance level below what is now required and increasing the level of self-crosslinking monomer does not enhance performance. Higher dry tensile strengths in a nonwoven product tends to impart stiffness or a hardness to the product and make it uncomfortable to the touch.